(1) Field of the Invention
The present invention relates to a method for optimizing a path of an optical network and to an optical transmission node for realizing path optimization, and more particularly, to a technique suitable for use in a main optical communication (WDM: Wavelength Division Multiplex) network.
(2) Description of the Related Art
A point-to-point network has become mainstream in a long-haul network. However, in accordance with increasing demand for lines and complication of the lines, there is growing necessity for mesh-type networking, in which importance is placed on a path.
The basic concept of a WDM network design that has become predominant in the field of the related-art network of a point-to-point type is to transmit all wavelengths from a certain node to another node [including add-drop (Add-drop) nodes provided at arbitrary points between the nodes]. To this end, regenerator nodes are disposed so as to maximize a “3R distance of a system” [an electronic regenerative relay (regeneration); namely, a distance over which wavelengths are transmitted without involvement of optical/electrical/optical conversion]. Once the 3R distance of the system and arrangement of the regenerator nodes have been determined, the arrangement of the regenerators is fixed even when a future increase in the number of paths arises, and re-design for additional paths is not required (is impossible).
A design, such as a 3R design (the arrangement of the regenerator nodes) of such a point-to-point-based system and dispersion compensation, or the like, is created on paper (with a tool) in accordance with span data (information about optical fibers between nodes, or the like) provided by the client before start-up of the system.
Specifically, in the design of a conventional WDM network, a necessity for regenerators of respective nodes/lack of such necessity and a dispersion compensation value are determined on the basis of information about an optical fiber of the network (a loss and dispersion in a transmission path) so as to maximize the “3R distance of the system,” on paper (with a tool) before installation of the system. As shown in, e.g., FIG. 9, in the conventional predominant point-to-point network, essentially all the paths employ the same starting and end points, and some of the paths are dropped (or added) at nodes provided between the starting point and the end point. Accordingly, in many cases, the number of regenerators (R, see a hatched section in FIG. 9) can be minimized by achieving the longest 3R distance for the system.
However, when the optical network of the overall system is of a mesh type and when the paths have become complicated, the design optimized to the 3R distance of the system involves placement of unwanted regenerators. An example mesh-type optical network is described in Patent Document 1 provided below.
Patent Document 1
JP-T-2002-504776
As shown in FIG. 10, when a mesh network is configured in accordance with the conventional design philosophy for optimizing the network to a 3R distance of a system, the mesh network becomes an aggregation of point-to-point systems (3R systems: refer to arrow 100 drawn in a heavy solid line) optimized to the 3R distance of the system. In FIG. 10, reference numerals 101 to 107 designate optical transmission nodes (hereinafter called “drop nodes”), each of which has the function of adding/dropping a WDM signal; 200 designates an optical relay node; 300 designates a wavelength multiplexing separation section; 400 designates a regenerator (R) (an O/E/O converter); and 500 designates a cross-connect section. Each of the drop nodes 101 to 107 is formed from a combination of one wavelength-multiplexing separation section 300, one regenerator 400, and one cross-connect section 500, the nodes being equal in number to input/output channels (WDM channels).
In this case, a path (b) set from the drop node 101 to the drop node 105 runs through a 3R system 100 which connects together the drop node 100 and the drop node 102. After having been temporarily terminated at the drop node 102 (where a regenerator has been arranged), the path is added to/dropped off from any position in the 3R system 100 that connects the drop node 103 and the drop node 105.
This configuration is adopted even when the distance (transmission path) from the drop node 101 to the drop node 105 may enable transmission without an electronic regenerative relay embodied by the regenerator 400. Since the design (dispersion compensation or the like) has been created so as to optimize the 3R distance of each system, transmission from the drop node 101 to the drop node 105 cannot be guaranteed.
In order to solve the problem, design that optimizes a 3R distance of each path (wavelength) rather than the 3R distance of the system is to be achieved. However, a plan for increasing the number of paths (wavelengths) or a path drop plan is not necessarily determined in a period of early design. For this reason, the conventional design technique, such as dispersion compensation or the like, optimized to the initially-fixed 3R system, fails to facilitate an optimum design in terms of future addition of paths.
When, at the beginning of an on-paper design, an attempt is made to create a design so as to optimize a 3R distance of a path, the number of regenerators can be curtailed as compared to that required for a design of the 3R distance of the system (i.e., only a design is made on a per-system basis in an early stage, and a re-design/re-setting is not required at the time of addition of paths). However, there is an increase in effort required to effect optimization design on a per-path basis from the period of initial setting and effort required to make a reset in response to increased paths.
Moreover, in the case of the mesh-type network, a method for selecting (determining) a path from a starting point to an end point also presents a challenge. Specifically, the shortest distance does not necessarily become optimum in terms of cost. Depending on conditions for a transmission path (a poor-quality optical fiber or the like), a regenerator can be required in view of performance requirements other than a distance. Therefore, in order to determine an optimal path, the number of regenerators provided in a path or the number of wavelengths used over the path must be taken into account.
Moreover, as a general problem of WDM, design of a WDM line itself poses heavy load on a client. Namely, in order to cause a WDM apparatus to exhibit its maximum performance, a channel design based on information about rigorous measurement of a transmission path (an optical fiber) is indispensable. However, in many cases, rigorous measurement information is not available at the time of planning for laying a line or purchasing equipment. Design and purchase of equipment are performed on the basis of rough data. Measurement or re-design of a transmission path is carried out before start-up of the apparatus. For these reasons, duplicative efforts are required for a design. In the worst case, a re-design will also require changes in articles.
The present invention has been conceived in view of the above-described drawbacks and aims at enabling design of a path so as to optimize not a 3R distance of a system but a 3R distance of a wavelength path, by means of enabling an optical network to autonomously transmit information about conditions for a transmission path between nodes, to thus select (determine) an optimal pathway (e.g., a pathway involving a minimum number of regenerators, or the like) to which a wavelength path is to be set.